The refractive index of an optical fiber is known to vary, within relatively narrow limits, gradually or in steps throughout its cross-section as a function of radius. A knowledge of this variation, known as the index profile, is important for a determination of some of the basic characteristics of a fiber such as its light-gathering efficiency, its light-guiding properties and its bandwidth.
Several techniques have already been developed for this purpose. Among them is the reflection method according to which a light beam is focused upon a substantially punctiform area (usually on the order of 1.mu. in diameter) of a fiber end by an optical objective through which reflected luminous energy is passed by suitable light-guiding means such as a beam splitter to a photodetector feeding an evaluator. As the projected light spot is displaced along a diametrical line of the fiber end confronting the objective, the evaluator calculates the refractive index from the reflected energy and plots it for different distances from the fiber axis; this calculation can be carried out with the aid of a reference value derived, advantageously via the aforementioned beam splitter, from the output of the light source.
Index-determining systems of this type have been described in an article by W. Eickhoff and E. Weidel entitled "Measuring Method for the Refractive Index Profile of Optical Glass Fibres", published March 1975 in Optical and Quantum Electronics, Vol. 7, No. 2, and in an article by Masahiro Ikeda, Mitsuhiro Tateda and Haruo Yoshikiyo entitled "Refractive Index Profile of a Graded Index Fiber: Measurement by a Reflection Method", published April 1975 in Applied Optics, Vol. 14, No. 4. Both these articles are referred to in a paper by B. Sordo and me entitled "Measurements of the Refractive Index Profile in Optical Fibres: Comparison between Different Techniques" which was presented at the Second European Conference on Optical-Fiber Communication, 20 to 30 September 1976, Paris; this paper proposes the use of a fluid (oil), of a refractive index closely approaching that of the fiber to be tested, in the space between the focusing objective and the confronting fiber end.
Even with the improvement last described, such an apparatus is not free from drawbacks. More particularly, spurious reflections at the objective itself tend to superimpose themselves upon the light reflected at the fiber end so as to falsify the evaluation results. These spurious reflections, despite their low absolute power, may have a significant influence upon the measured values since the refractive index varying along a continuous (e.g. parabolic) curve may change from one area of illumination to the next by increments on the order of 0.001 whose detection requires the sensing of differences in reflected energy amounting to a fraction of one percent. Thus, a system using an index-matching fluid as discussed above will give rise to reflected energy at the fiber end amounting to about 10.sup.-4 times the incident energy which would be overshadowed by the spurious reflections at the objective surfaces; while this problem could be alleviated by the use of lenses with antireflection coatings, the latter would have to be specifically designed for the luminous wavelength employed and would be difficult to apply to an existing, commercially available objective. Moreover, with coherent light emitted by a gas laser serving as the source, the spurious reflected rays will interfere with the incident rays and give rise to a time-varying power distribution resulting in an unstable output signal.